Syntax

Members

The source blend operation. Currently, the only source and destination blend operation that has been defined is AC_SRC_OVER. For details, see the following Remarks section.

BlendFlags

Must be zero.

SourceConstantAlpha

Specifies an alpha transparency value to be used on the entire source bitmap. The SourceConstantAlpha value is combined with any per-pixel alpha values in the source bitmap. If you set SourceConstantAlpha to 0, it is assumed that your image is transparent. Set the SourceConstantAlpha value to 255 (opaque) when you only want to use per-pixel alpha values.

AlphaFormat

This member controls the way the source and destination bitmaps are interpreted. AlphaFormat has the following value.

Value

Meaning

AC_SRC_ALPHA

This flag is set when the bitmap has an Alpha channel (that is, per-pixel alpha). Note that the APIs use premultiplied alpha, which means that the red, green and blue channel values in the bitmap must be premultiplied with the alpha channel value. For example, if the alpha channel value is x, the red, green and blue channels must be multiplied by x and divided by 0xff prior to the call.

Remarks

When the AlphaFormat member is AC_SRC_ALPHA, the source bitmap must be 32 bpp. If it is not, the AlphaBlend function will fail.

When the BlendOp member is AC_SRC_OVER, the source bitmap is placed over the destination bitmap based on the alpha values of the source pixels.

If the source bitmap has no per-pixel alpha value (that is, AC_SRC_ALPHA is not set), the SourceConstantAlpha value determines the blend of the source and destination bitmaps, as shown in the following table. Note that SCA is used for SourceConstantAlpha here. Also, SCA is divided by 255 because it has a value that ranges from 0 to 255.

Dst.Red

= Src.Red * (SCA/255.0)

+ Dst.Red * (1.0 - (SCA/255.0))

Dst.Green

= Src.Green * (SCA/255.0)

+ Dst.Green * (1.0 - (SCA/255.0))

Dst.Blue

= Src.Blue * (SCA/255.0)

+ Dst.Blue * (1.0 - (SCA/255.0))

If the destination bitmap has an alpha channel, then the blend is as follows.

Dst.Alpha

= Src.Alpha * (SCA/255.0)

+ Dst.Alpha * (1.0 - (SCA/255.0))

If the source bitmap does not use SourceConstantAlpha (that is, it equals 0xFF), the per-pixel alpha determines the blend of the source and destination bitmaps, as shown in the following table.

Dst.Red

= Src.Red

+ (1 - Src.Alpha) * Dst.Red

Dst.Green

= Src.Green

+ (1 - Src.Alpha) * Dst.Green

Dst.Blue

= Src.Blue

+ (1 - Src.Alpha) * Dst.Blue

If the destination bitmap has an alpha channel, then the blend is as follows.

Dest.alpha

= Src.Alpha

+ (1 - SrcAlpha) * Dst.Alpha

If the source has both the SourceConstantAlpha (that is, it is not 0xFF) and per-pixel alpha, the source is pre-multiplied by the SourceConstantAlpha and then the blend is based on the per-pixel alpha. The following tables show this. Note that SourceConstantAlpha is divided by 255 because it has a value that ranges from 0 to 255.

Src.Red

= Src.Red

* SourceConstantAlpha / 255.0;

Src.Green

= Src.Green

* SourceConstantAlpha / 255.0;

Src.Blue

= Src.Blue

* SourceConstantAlpha / 255.0;

Src.Alpha

= Src.Alpha

* SourceConstantAlpha / 255.0;

Dst.Red

= Src.Red

+ (1 - Src.Alpha) * Dst.Red

Dst.Green

= Src.Green

+ (1 - Src.Alpha) * Dst.Green

Dst.Blue

= Src.Blue

+ (1 - Src.Alpha) * Dst.Blue

Dst.Alpha

= Src.Alpha

+ (1 - Src.Alpha) * Dst.Alpha

Requirements

See Also

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